The health and appearance of a one's teeth is one of the main factors determining one's general health and self image, which is important for digestion, psychological, social and sexual well being. Generally, the condition of the teeth depends upon genetic, lifestyle, dietary, environmental and other factors. Human teeth are exposed to mechanical and chemical processes associated with food and beverage consumption, as well as the impact of bacteria and other natural and artificial substances and objects on a daily basis. In the modern world with its processed foods and sugary diets, teeth can be rapidly discolored, damaged, worn, eroded and even lost without daily oral hygiene and regular inspection and maintenance. Unlike other human tissues, the tooth enamel does not contain mechanisms for self-protection and rejuvenation. The enamel normally can restore itself by a remineralization process with the necessary minerals and action obtained from saliva. There is a continuous demineralization/remineralization process, which restores the health of the enamel tissue, damaged by the actions described above. The past several decades have seen the introduction of many new methods improving strength of the enamel and aiding its remineralization. Such methods include, but are not limited to, fluoridation of water, using fluoridated toothpastes containing amorphous calcium phosphate, using more effective toothbrushes, including electrical brushes, using new types of rinses, of teeth using peroxide-based agents has become increasingly popular. As a result, here has been a significant decrease in tooth loss due to caries and an improvement of teeth appearance in the countries where such methods are available.
In the United States, however, 85% of population still suffers from caries and over 30% of adults are not satisfied with the cosmetic appearance of their teeth. This situation is significantly worse in the countries with no water fluoridation. Therefore, the development of new treatment for tooth protection and rejuvenation is a very desirable objective.
Tooth Structure
Human teeth serve several functions, including chewing, aiding in speech, and the perception of beauty and facial harmony. A human tooth consists of three sequential layers of tissues: (1) the hard, highly mineralized tissue, the “enamel”, supported by the less mineralized and vital connective tissue, (2) the “dentin”, which is formed from and supported by soft, connective tissue, and (3) the “dental pulp” or the “pulp”. The pulp consists of sensitive tissue containing blood vessels, nerve fibers, specialized cells and pulpal fluid. The dentin, which surrounds the dental pulp, forms the major part of the tooth. It is dense bonelike tissue consisting of 70% inorganic material, 20% organic material, and 10% water by weight. The enamel, which surrounds coronal dentine, consists of 96% inorganic, 1% organic material and 3% water by weight. The inorganic material is called hydroxyapatite, a substance also found in bone and dentine. A tightly packed mass of apatite crystals forms the basic structural unit of enamel, called the “enamel rod” or “enamel prism.” It is shaped like a keyhole and has an average width of 5 μm. Its width is determined by the local enamel thickness, with a maximum of approximately 2.5 mm. Rods run from the dento-enamel junction perpendicularly to the outer enamel surface and are maintained in rows. Neighboring rods are separated from each other by 0.1-0.2 μm wide prism sheaths. The enamel rod consists almost entirely of hydroxyapatite, whereas the prism sheaths are made up largely of organic material comprised of amelogenin polypeptide and non-amelogenin proteins. The mineral component of enamel is an apatite like crystal, which has the formula of Aio(BO4)6X2, where A is Ca, Cr, Ba, Cd, B is P, As, Si, and X is F, OH, ClCO2. The dominant formula of enamel apatite is an ideal hydroxyapatite Ca10(PO4)6(OH)2 with the Ca/P ratio of 1.67. In addition to hydroxyapatite (−75%), carbide apatite (−3-20%), chlorine apatite (<<4%), fluorine apatite (<<0.5%) are also present in enamel. Apatite is formed in hexagonal micro crystals with a size of (14-46) nm×(27-78) nm. These crystals have the typical crystal defect in the lattice arrangement including shifted, disrupted, and curved lattice planes. Defective lattices in the boundary between crystals are fused with each other. In carious lesions, mineral dissolution begins in the crystal lattice defects. The micro crystals in enamel are surrounded by a water shell, which makes enamel transparent for some ions. The main requirements for healthy enamel are mechanical hardness, wear resistance, and caries resistance (which is essentially acid resistance). In addition, the esthetic appearance of especially the anterior teeth has become of significant importance in today's appearance conscious society.
Remineralization and Demineralization of Hard Tissue
The process of the dissolution of enamel is called demineralization. It is the result of the interaction of the enamel components with the acid, produced by the bacterial action of plaque and various foods, as well as by the consumption of acidic beverages, such as fruit juices, wine and some sports and carbonated drinks. The decrease in a pH results in the dissolution of Ca and P ions into the saliva. The solubility in acid of different types of the apatite found in the enamel varies significantly. For example, the solubility of carbonate apatite in an acid with a given pH is approximately an order of magnitude greater than that of hydroxyapatite, which, in turn, is an order of magnitude greater than that of fluorapatite.
The reverse process is called remineralization, which is facilitated by some or all of the following mechanisms. Human saliva contains calcium and phosphate in a supersaturated state, which can remineralize hydroxyapatite crystals lost during demineralization. This is the fundamental process in the prevention of enamel loss. Under normal conditions, there is a balance between demineralization and remineralization. The remineralizing ability of saliva is a typical example of the natural tooth rejuvenation mechanism. The remineralization process can also be initiated by controlling an oral fluid. The resistance of teeth to an acid attack can be increased and such methods as the use of fluoride in toothpastes and community water supplies have been known for many years. F ions from compounds, such as NaF and SnF2, replace some of the OH— ions in apatite during the remineralization process. The modified enamel substance, called fluorapatite, is more resistant to acid than hydroxyapatite. Amorphous calcium phosphate (CaPO4) or ACP, is another compound used to promote enamel remineralization. As the pH falls, ACP dissociates to form calcium and phosphate ions, thereby minimizing the drop in the pH and limiting demineralization. Since ACP can act as a reservoir for calcium and phosphate ions and maintain these ions in a state of supersaturation with respect to enamel, ACP decreases the process of demineralization and promotes remineralization. Remineralized complexes consisting of Ca and F have been suggested as additives to strips and filling material.
Acid Etching
Acid etching or enamel conditioning has a widespread use in clinical practice. It is most frequently used in bonding of resin materials. Different types and concentrations of acid may be used. Of these, 30-40% phosphoric acid with an application time of up to 60 seconds is the one most frequently used. Another, less frequent acid application is the removal of the superficial enamel stains resulting from the developmental disturbances of the enamel, such as excessive intake of fluoride. Reported uses involve 18% and 37% hydrochloric acid applied for up to 25 seconds.
Acid etching and partial demineralization of apatite crystals leads to the high porosity of exposed surfaces, which makes such surfaces better suited for bonding of the restorative and adhesive materials. Three distinct acid etching patterns can be distinguished. A type I pattern is the one where the enamel rod cores are preferentially removed. In the type II pattern mostly prism sheaths are removed, while the rod cores remain intact. The type III pattern is characterized by irregular and indiscriminate etching. The cause for the described differences is unclear. One explanation may be that the differences in the orientation of the c-axis of apatite micro crystals relative to the enamel surface cause the differences in the etching patterns, because the solubility of the apatite in the c-axis configuration is lower than that of the crystals in the perpendicular direction.
Acid etching of hard tissue is a cause of the enamel loss and the decrease of mechanical hardness and wear resistance. In addition, acid etching of the superficial enamel layer, which is the most resistant to acid attack, can accelerate the growth of a carious lesion. For this reason, acid is used in dentistry mainly for the treatment of hard tissues to facilitate adhesion of tooth colored restorative materials to such hard tissues. In low concentrations, an acid (pH>5) is used as an addition to peroxide bleaching agents and some rinses and toothpastes for the stabilization of various ingredients. Dentists recommend limiting the use of acidic beverages and foods. Most foods and beverages have a pH of 2.5 or more, usually between 4 and 7.
Cosmetic Appearance of Teeth
The appearance of a tooth is important in today's society. Anterior teeth play the main role in this appearance. Among the many factors, which determine the appearance of a human tooth, the most important are (a) “color”, (b) “gloss”, and (c) “translucency”:                (a) “Color” can be described as the result of the interaction of a tooth with light, including reflection, absorption and transmission. Light in general is electromagnetic radiation, and the light visible to the human eye is characterized by the wavelength within the spectral range from about 400 nm to about 800 nm. Different wavelengths are associated with different hues, such as blue, represented by a wavelength of 470 nm, green by 540 nm, and red by 670 nm. White light contains a mixture of all of the wavelengths and is similar to sunlight. Human enamel may selectively reflect only the wavelengths from a portion of the spectrum, while absorbing and transmitting the other portions. The reflected portion determines, in part, the tooth's color. For example, a yellow enamel surface reflects mostly the yellow portion of the light spectrum and partly absorbs the blue incident wavelengths. A black surface absorbs the light of entire visible spectrum and reflects none. A white surface reflects the light of all incident wavelengths in uniform fashion. In addition to color of the surface exhibited due to reflection, a portion of the incident light may be transmitted to the dentin-pulp complex, where a portion of the transmitted light is absorbed by the blood (400-600 nm) and another portion of the incident light is reflected, affecting the tooth's color.        (b) Incident light may be reflected in a diffused or specular fashion. In specular reflection, the angle of incidence of a light beam is equal to the angle of reflection, resulting in a lustrous appearance, said to have high “gloss”. This reflection takes place only from well-polished enamel surfaces with micro pores smaller than the wavelength of the incident light. In diffuse reflection, the reflected light is scattered in all directions, resulting in a decrease in gloss. High gloss is usually associated with a smooth enamel surface. In addition, a significant portion of the diffused light is reflected from the body of the enamel, the dento-enamel junction, the dentine and the pulp.        (c) “Translucency” is an optical property of an object, which allows it to transmit or scatter incident light. A highly, translucent tissue transmits most of the incident light, resulting in a more transparent and lighter colored appearance. An increase in scattering within the tissue leads to a decrease in its translucency and an increase in its opacity. Light scattering is the result of scattering at the centers within the tissue. Light scattering is affected by the size, shape, and number of scattering centers, as well as by the difference in the refractive indices between different components of the tooth.        
Tooth discoloration can be classified according to the location of a stain, which may be extrinsic or intrinsic:    1. Extrinsic stains are mainly caused by the daily intake of substances, such as foods and beverages, and/or the by the use of tobacco products, etc. These substances tend to adhere to the enamel's structure and thereby discolor the teeth and/or reduce their whiteness. Most extrinsic stains are accumulated in the plaque, pellicle, tartar and the superficial enamel layer with a thickness of up to a dozen micrometers. Extrinsic discoloration typically affects the tooth enamel surface and may be classified according to its origin, and whether it is “non-metallic” or “metallic”:            a) “Metallic” stains are formed as a result of exposing the enamel surface to metal salts. Such exposure can occur either via consumption of medicines containing such salts or via occupational exposure to metals, such as that found among foundry workers.        b) “Non-metallic” stains are formed on the enamel surface deposits as a result of consuming various dietary products, beverages, tobacco, mouthwashes and medicaments.            2. Over a period of years extrinsic stains may penetrate the enamel layer and gradually cause intrinsic discolorations. “Intrinsic stains” is the term used for stains, which have penetrated the tooth structure (i.e. discoloration within the tooth matrix). Intrinsic discoloration is located beneath the enamel surface and occurs as a result of changes in the physical properties or a structural composition of the tooth tissues. The exact location of a stain within the enamel has not been known with certainty. Intrinsic discoloration may be classified according to its cause, with the following types generally recognized:            (a) “Ageing” is frequently associated with thinning of the enamel and an increase in its translucency. The increase makes the dentin-pulp complex more visible, leading to an overall darkening of the teeth.        (b) “Alkaptonuria” is a condition affecting the permanent dentition, leading to brown discoloration as a result of an incomplete metabolism of tyrosine and phenylalanine.        (c) “Amelogenesis imperfecta” is a hereditary condition, where the enamel calcification is disrupted during the tooth formation, resulting in a discoloration varying from the mild “white-spot” lesions to the hard enamel with the yellow-brown appearance.        (d) “Congenital erythropoietic porphyria” is a metabolic disorder resulting from an error in the porphyrin metabolism, leading to the accumulation of porphyrins in the dentition and its red-brown discoloration.        (e) “Congenital hyperbilirubinaemia” is caused by the breakdown products of haemolysis, resulting in the yellow-green discoloration.        (f) “Dentinal dysplasias” are hereditary conditions where the primary and secondary dentition is of a normal shape and form, but may have an amber translucency.        (g) “Dentinogenesis imperfecta” is a dentine defect, which occurs genetically or through environmental influences, resulting in bluish or brown discolorations.        (h) “Enamel hypoplasia” is most likely to occur following a trauma or infection in the primary dentition. This defect is frequently accompanied by pitting or grooving, which is predisposed to extrinsic staining of the enamel, often then becoming internalized.        (i) “Fluorosis” results from an excessive intake of fluoride found in the water supply, mouthwashes, toothpastes and certain types of medication. Fluoride interacts with the enamel's hydroxyapatite crystals, resulting in brown-black stains.        (j) “Pulpal hemorrhage” is caused by a severe tooth trauma and results in a purple-pink discoloration caused by the blood pigments.        (k) “Root resorption” begins at the root surface, resulting in a pink appearance at the cemento-enamel junction.        (l) “Systematic syndromes” is represented by the defects in the enamel formation, occurring as a result of clinical syndromes, such as Vitamin D dependent rickets, epidermolysis bullosa and pseudo-hypoparathyroidism.        (m) “Tetracycline staining” is caused by systematic administration of tetracycline antibiotics during the tooth development. Tetracycline forms complexes with the calcium ions of the hydroxyapatite crystals within the dentine, resulting in a yellowish or brown-gray appearance.        
An understanding of the reasons for enamel discoloration is helpful for the in-depth understanding of the proposed method and device for tooth whitening. A child's or adolescent's teeth are much whiter than those of an adult, as with age, teeth discolor. This discoloration is caused by the consumption of foods and beverages containing natural dyes, smoking and other external causes. An additional cause, independent of these, is the structure of the tooth enamel, which is affected by aging. At a younger age teeth are whiter, because enamel has a high porosity and its prisms are randomly oriented. A material with such a structure scatters light very well. The better the scattering properties of the enamel, the whiter its appearance. Over its lifetime, the enamel hardens, the size of the prisms increases and their orientation relative to each other becomes more orderly. These changes cause the enamel to gradually loose its scattering properties and become more transparent, allowing light to penetrate to the underlying dentin, and be scattered and reflected, resulting in the observer's seeing a color influenced by the color of the dentin, which is more yellow. For humans this process occurs from about the age of 40. Ignoring external factors (oral hygiene, coffee, tea, wine and tobacco consumption, and trauma, etc), the objective of tooth whitening relates to whitening enamel and reconstructing its scattering properties, mostly in the superficial layer. Existing whitening methods, such as those utilizing hydrogen peroxide, do not address this problem effectively because they mostly bleach the superficial stains.
Tooth Rejuvenation
Tooth rejuvenation is one of the most important parts of preventive and esthetic dentistry. As explained above, it can be a part of the natural process, facilitated by the saliva. However, in many cases the natural role of the saliva may not be enough to keep a tooth from degradation. Several methods aimed to enhance tooth rejuvenation exist. Most are focused on the improvement of one the components of tooth rejuvenation, and do not provide a complete solution. Such methods are: water fluoridation, mouth rinses, gels and strips, tooth brushing, professional oral cleaning, tooth whitening, tooth coating, tooth surface laser modification. These methods are described below in more detail.
1. Water fluoridation contributes to the formation of fluorapatite in the external layer of the enamel. Fluoride in water plays several roles in the prevention of dental caries, such as the inhibition of acid production in plaque, the enhancement of remineralization of carious lesions and strengthening the enamel against an acid attack through the formation of the fluorapatite (Caio(PO4)6F2) this effect takes place at low concentrations of fluoride. High concentrations of fluoride can cause the formation OfCaF2 and the destruction of tooth structure.
2. Mouth rinses are mainly used for bacterial reduction. Some additives, such as the casein phosphopeptide-amorphous calcium phosphate nano-complexes, have been proven to be effective in the remineralization process.
3. Different types of gels and strips and have been shown to provide an antibacterial effect. A gel, containing fluoride, calcium and phosphate ions, has been shown to be effective in the remineralization process. Preliminary treatment of enamel with low acid concentrations enhances the effect of the fluoride treatment. Gels or strips may also include peroxide for tooth whitening.
4. Tooth brushing and flossing are the most important forms of preventing tooth stains and destruction of teeth, since they are daily regimens. The mechanical cleaning of the teeth removes a biofilm, prevents/decreases the build up of tartar and decreases acid production by bacteria. It also enhances the access of saliva to the enamel, in the process improving the chances for remineralization. In addition, toothpastes often contain antibacterial, remineralizing and whitening components.
5. Professional oral cleaning in the dental office provides additional benefits to the methods of tooth brushing and flossing, such as the removal of supra and subgingival plaque and calculus, plaque detection, and application of caries-preventing agents. The treatment typically involves the procedures, such as scaling and polishing of teeth and subgingival currettage, resulting in a more effective method of preventing of periodontal or other dental diseases, as well as an overall aesthetic improvement in the appearance of teeth and gums. Plaque detection and the application of the caries-preventing agents may also be performed by the health professional as an aid to home care and remineralization. However, this treatment is not capable of removing intrinsic and deep extrinsic stains.
6. Tooth whitening has been one of the fastest growing tooth rejuvenation procedures during the last decade. Prior to tooth whitening, a correct diagnosis of the cause of the discoloration needs to be made. Certain extrinsic stains, which occur on the surface or subsurface of the teeth, can be removed by mechanical means. Not all extrinsic stains can be removed mechanically. Some stains are better removed with the whitening agents, which inhibit non-enzymatic browning reactions. Intrinsic stains are located in the tooth matrix and cannot be removed by intense mechanical brushing of the teeth. Removal of intrinsic stains calls for the whitening agents capable of penetrating into the tooth structure. Three types of whitening treatments are available: a) “mechanical abrasion”, b) “acid abrasion”, and c) “peroxide bleaching”.
a) Mechanical abrasion is used for the removal of superficial extrinsic stains, mostly accumulated in tooth plaque, pellicle and tartar. Extrinsic stain removal is is achieved manually and mechanically by machine scaling followed by mechanical brushing with abrasive cleansing agents. The brushing step is performed with either a regular toothbrush or rotary instrumentation. The cleansing agents usually contain abrasives and surfactants, typically found in modern toothpastes, or dental pumice.
b) “Acid/abrasion” whitening involves the removal of a stained tooth structure and tooth stains simultaneously to a depth of approximately 100 μm. The first published tooth whitening technique, reported by Chaple in 1877, used oxalic acid. Modern techniques involve etching of the enamel surface by an 18% hydrochloric acid solution, followed by mechanical abrasion. This technique has been suggested for the removal of brown stains associated with an excessive fluoride intake. This technique is destructive and time consuming, so the concerns are raised about the safety of the soft tissue and damage to it due to the low pH of the acid used. Another drawback of this technique is the lack of predictability of the results, because it is typically difficult for a clinician to ascertain the probable depth of the stain, which significantly limits the use of the technique in everyday practice.
c) The first report of “peroxide bleaching” was published by Harlan in 1884. Although many whitening agents have subsequently been suggested, peroxides remain the most commonly used teeth bleaching compounds. Peroxide bleaching works by oxidation—the chemical process in which hydrogen peroxide (H2O2) releases free radicals (HO2+O2), with unpaired electrons, which are given up to the bleached substance, oxidizing it and making it lighter in color. In dental bleaching, hydrogen peroxide diffuses through the organic matrix of the enamel and oxidizes the organic material located in the prism sheaths. Peroxide whitening techniques are usually divided into two main categories: “non-vital” and “vital”:
The non-vital techniques (treatment of a tooth with a non-vital or endodontically treated pulp) often provide very good results, but they have limitations and potential hazards. These limitations and hazards include a potential root resorption if the bleaching agent is placed below the coronal portion of the tooth. One non-vital whitening technique uses sodium perborate and 35% hydrogen peroxide as the active ingredient.
Products sold for vital whitening techniques can be divided into three main groups: (i) “in-office” whitening products, (ii) dentist prescribed, home-applied whitening products, and (iii) over-the-counter whitening kits and toothpastes.                i. One of the most commonly used “in-office” techniques combines the use of 35% hydrogen peroxide with heat and light treatment to speed up the oxidation reaction (i.e. the removal of stains),        ii. Another method, using a “dentist prescribed, home-applied” whitening product, involves the use of 10% urea peroxide (carbamide peroxide). An individually fabricated mouth tray is constructed for a patient, the whitening agent is placed into this tray which is then worn by the patient for an appropriate period of time.        iii. Whitening kits can be used for whitening teeth and include products, such as toothpastes and mouthwashes having from 3% to 6% hydrogen peroxide. Such whitening kits are sold directly to consumers without a prescription from a dentist.        
Generally, there are three variables that can be varied to control the rate of whitening during the procedure utilizing the peroxide agents. The first variable is a concentration of the peroxide. In order to make the procedure occur within a reasonable period of time, concentrations of peroxide equivalent as high as 35 percent by weight are used. The peroxide-based whitening composition can be in a liquid, paste or gel form, with the gel being the most popular. The second variable is the exposure time, i.e., the time during which the tooth is exposed to the peroxide. The third variable is a pH of the peroxide mixture.
Peroxide tooth whiteners with a higher pH are more effective than the identical ones with a lower pH. Unfortunately, a higher pH also means the decreased peroxide stability. Consequently, none of the present tooth whitening materials have a pH much above neutral, while most are actually acidic. The only exceptions are those materials requiring an addition of an alkalinity adjuster immediately prior to use, but this approach has little consumer or professional appeal because of the complex handling and preparation procedures involved.
Another problem in designing a desirable tooth whitening product is a lack of a good gelling material which can be used at the higher pH ranges. Virtually all of the current stable tooth-whitening gels use a carbomer matrix. Carbomer in its initial gelled form has a low pH. An increase in pH leads to a loss of viscosity and stability of the carbomer, requiring great skill and effort to keep the material useful above a neutral pH. As a result, the only single-tube, high-concentration peroxide gel product to ever reach the marketplace (Ultradent of Salt Lake City, Utah) is so sensitive to destabilization by heat exposure that the manufacturer refuses to ship during certain weather conditions or over a weekend. Once received by a dentist, the material needs to be refrigerated at all times, or its efficacy is at risk. An end user is left with a product, which has unpredictable and unsatisfactory characteristics, since its effectiveness can be completely destroyed by a common uncontrollable event, such as a slow shipment.
Thus, the efficiency of whitening teeth, the safety of the procedure and the stability and shelf life of whitening agents present significant obstacles to their successful use. A further problem is that effective concentrations of hydrogen peroxide exceed the concentration limits allowed in certain countries. Products comprising a low concentration of whitening agents, such as hydrogen peroxide, are considered to have a slow whitening effect. Therefore, there is a need for providing safe tooth whitening compositions, which do not contain harmful concentrations of peroxide. It is further desirable that such tooth whitening materials be used as the components in conventional oral care products for “home-use”. To date, tooth whitening has been accomplished by using peroxide as the bleaching agent. When peroxides decompose, they release oxygen, which denatures the proteins, which act as pigments. The main problem in using them is that the required high concentrations of peroxide are less safe when those used intraorally. A further problem is that the peroxides are unstable and have a short shelf life.
7. Coating the external surface of the tooth or other hard tissues is one of the most effective methods of changing its appearance and protecting it from an acid attack. Several light cured compounds for the protection of the enamel surface, such as BISCOVER™, have been proposed. Such methods are either very destructive (veneers), or discolor and wear rapidly, thereby losing their effect (polymer-based coating materials and flowable resin composites).
8. Teeth function in an environment of mechanical, chemical and thermal stress. With normal chewing, a modest stress of 20 MPa is applied to the tooth more than 1000 times a day. Occasional stress can be up to 100 MPa. This cyclic loading occurs in a water-based fluid environment that can have a pH from 0.5 to 8 and the temperature variations of 50° C. Many different restorative materials have been developed, designed to retain their strength and properties in an aggressive environment (for example, ceramic-based porous alumina infiltrated with lanthanum aluminosilicate glass, or porous zirconia later infiltrated with glass). Porcelain, the most popular material, has excellent color properties, but is brittle and relatively easily fractured unless it is reinforced or strengthened. Porcelain restoration treatment also destroys the tooth structure, since it usually requires tooth preparation and is expensive and time consuming. These restorative materials are used for crowns or veneers and, if done properly, provide excellent esthetic appearance and prevent caries. However, the risk of recurrent caries still exists. Since any destruction of the tooth substance is harmful, clinicians have been attempting to develop non-destructive, or minimally destructive methods for tooth restoration. One such area of research involves the use of lasers.
Tooth or other hard tissues' surface laser modification is a method of selectively heating the superficial layer of hard tissue to high temperatures below or above the melting temperature of its mineral components. After cooling, a layer of newly modified material is created on the tooth surface. This layer can be more resistant to an acid attack, have a lower porosity, higher hardness and wear resistance than the original enamel or dentine. Such selective heating can be achieved in the oral cavity using a laser. The first laser modification of enamel with increased acid resistance was demonstrated in 1964. Subsequently, other lasers have been studied: the UV excimer laser (ArF laser: 0.193 μm, the KrF: 248,308 μm), the solid-state laser (Ruby: 0.69 μm, the Nd:YAG 1.06 μm, the Ho:YAG 2.06 μm, the Er:YAG 2.9 μm) and gas lasers (CO2: 9.6 μm, 10.6 μm). Heating of the enamel up to a temperature of 400-600° C. leads to a significant loss of carbonate and an increase in the enamel's acid resistance. Further heating to the melting temperature (800-1400° C.) of the mineral components of the enamel, but below ablation thresholds, induces a recrystallization process forming a new structure of the superficial layer with better mechanical and acid resistance properties. This effect was demonstrated for the sealing of early pit fissure caries. A 5 min fluoride treatment in carious-like enamel (1.23% acidulated phosphate fluoride gel, pH=4), followed by a laser treatment with a CO2 laser (9.6 μm wavelength, 1 J/cm2 fluence, 2 μs pulsewidth) dramatically increases the fluoride content in 1 μm of the superficial layer of enamel and significantly increases its acid resistance. Successful tooth surface laser modification requires precise adjustment of laser parameters. Most studies of the tooth surface laser modification show such side effects as carbonization, tooth darkening, crack formation in the modified enamel layer, and/or instability to thermocycling. In addition, the risk of overheating the tooth pulp exists. Finally, tooth surface laser modification has not been used in daily dental practice and no such product is currently available on the market.